To apply a continuous cast product to a round steel billet which is used for manufacturing a high Cr steel (steel containing a large content of Cr) such as a 13Cr steel, it is desirable that the continuous cast product for round steel billet has sound inner quality comparable to inner quality of a blooming mill product for round steel billet.
In a continuous casting process, in general, segregation occurs in the steel billet due to concentrated molten steel which remains in an axial core area of the steel billet (indicating a circle having a radius of (D/2)×0.2 about an axis in a cross section of the steel billet having an outer diameter D and an area inside the circle). Also porosity is generated in the steel billet due to shrinkage of the finally solidified axial core area. Accordingly, it is difficult for the round steel billet by the continuous casting process to have sound inner quality comparable to inner quality of the round steel billet by the blooming mill process. Particularly, the round steel billet used for manufacturing a seamless steel pipe or tube by roll piercing such as Mannesmann piercing is required to have sufficient workability. Accordingly, to apply the continuous cast product to the round steel billet, it is necessary to take measures to decrease segregation and porosity in the axial core area as much as possible.
As one of the above-mentioned measures, for example, there has been known a method which reduces a cross-section area of the cast product by adding rolling reduction to the unsolidified area in the inside of the cast product using rolls having a diameter 2 to 5 times as large as a thickness of the cast product, that is, the bloom or the billet during a terminal period of solidification in the course of continuous casting and, at the same time, by eliminating unsolidified molten steel in which impurity elements are concentrated from the axial core area of the cast product (patent literature 1, for example).
As an another countermeasure, there has been known a method where the cast product which is completely solidified is formed to have a predetermined cross-section shape by applying roll forming following the above-mentioned rolling reduction applied to the unsolidified area and, in such a stage, preferably, the surface of the cast product is cooled with a predetermined water volume from the completion of the rolling reduction to the starting of the roll forming (patent literature 2, for example).
On the other hand, there has been known a method where quality of the axial core area of the cast product is enhanced by controlling a secondary cooling condition of the cast product in the course of continuous casting within a specified range with respect to a steel having a specified chemical composition (patent literature 3, patent literature 4, patent literature 5, and the like, for example). In patent literature 4, casting speed is also specified. Further, in patent literature 5, it is described that electromagnetic stirring may be applied to the unsolidified area of the cast product.